This invention relates to papermakers' fabrics and especially to papermakers' fabrics for the forming section of a papermaking machine.
In the conventional papermaking process, a water slurry or suspension of cellulose fibers, known as the paper "stock", is fed onto the top of the upper run of a traveling endless forming belt. The forming belt provides a papermaking surface and operates as a filter to separate the cellulosic fibers from the aqueous medium to form a wet paper web. In forming the paper web, the forming belt serves as a filter element to separate the aqueous medium from the cellulosic fibers by providing for the drainage of the aqueous medium through its mesh openings, also known as drainage holes, by vacuum means or the like located on the drainage side of the fabric.
After leaving the forming medium, the somewhat self-supporting paper web is transferred to the press section of the machine and onto a press felt, where still more of its water content is removed by passing it through a series of pressure nips formed by cooperating press rolls, these press rolls serving to compact the web as well.
Subsequently, the paper web is transferred to a dryer section where it is passed about and held in heat transfer relation with a series of heated, generally cylindrical rolls to remove still further amounts of water therefrom.
Over the years, papermakers have sought improvements in the forming fabric, not only with respect to the operating life of the fabric, but also with respect to the quality of the paper sheet produced on it. Triple layer fabrics were introduced for this purpose. The triple layer fabric has two generally distinct surfaces. The top surface is one integral fabric structure designed specifically for papermaking to achieve the best possible sheet quality and machine efficiency. This top fabric is manufactured as an integral part of a woven structure with a completely separate bottom fabric designed specifically for mechanical stability and fabric life. The purpose of triple layer fabric development is to eliminate the compromises which exist with both single and double layer forming fabrics so that papermakers can produce the best possible paper sheet for top quality at reduced cost without sacrificing the wear characteristics of the papermakers' fabric.
References known in the art have described these so called triple layer fabrics with additional binder yarns in the warp direction and in the weft direction. Interconnection of the two fabric layers by a binder warp, however, has the drawback that during weaving the warp yarn is under tension so that it influences the structure on the paper side. Additionally, when a triple layer fabric is woven flat and made endless by means of a woven seam, the binder warp in the final screen extends in the longitudinal direction. Since the fabric is lengthened during thermosetting in the heating zone, the warp threads are again subject to high working tension. Owing to the fact that the weft threads of the lower layer are substantially thicker and stiffer, the tension of the binder warp affects nearly exclusively the finer threads of the upper layer. Thus, the binder warp pulls the fine weft threads of the upper layer deep into the fabric at the binding points, thereby causing distortions in the uniformity of the surface.
Accordingly, triple layer fabrics are in practice joined together by weft binder yarns. This solution is less than ideal, however, because the movement of the individual fabric layers relative to each other is unrestrained with the binder weft yarns. Delamination of these triple layer fabrics becomes a problem, reducing the fabric's life.